Optional ROS package (and catkin) dependencies

Is there (going to be) support for specifying optional dependencies for ROS packages in both package.xml and CMakeLists.txt? Rationale: I do some optional resource generation during the build process (optional robot parts), and then I want to merge all these generated resources in a common resource. So this common resource needs to be built only after the resource generation for the parts has finished. But I still need to preserve the optionality of the various parts.